Printing inks for use in flexographic and gravure printing have generally, for the last 60 years and almost predominantly for the earlier years in this period, utilized volatile solvent-based ink formulations following the invention by Adolph Weiss in the mid-1920's of the closed ink fountain (U.S. Pat. No. 1,631,169) which literally revolutionized rotogravure printing by opening the way to use much faster drying inks and thus much higher printing speeds.
Increased awareness of the detrimental effects of the rapid sweep of the industrialized world for power, money and higher levels of the "standard of living" on the environment and health and safety of all living things has, as we all know, led to increased governmental restrictions on all facets of our economy. Among these have been severe limitations on what can be vented to the atmosphere and in the context of this invention, on the amount of organic solvent vapor which can be vented. This has challenged those in the field of printing inks to lower and probably eventually totally eliminate any organic solvents in all printing inks and pastes. The response to this challenge has been the development of solventless inks; low-smoke low-odor inks, which can be used to replace the heatset inks; air-drying inks covered with a protective layer of alcohol-soluble polymer; thermally-catalyzed inks, which use blocked acid catalysts that become unblocked at high temperature; inks cured by infrared, ultraviolet light, and electron beam radiation; and water-based inks.
Solvent incineration systems have been developed to burn off the emitted solvents; however, these require additional fuel (natural gas or oil) and high temperatures. Solvent recovery units have also been developed to absorb, condense or otherwise recover the solvent; however, the solvent recovery is only partial, and some solvent escapes to the atmosphere. Solvent incineration has been used to burn off solvents from the paste inks used for lithographic and letterpress printing, and solvent recovery has been used to recover the solvent from the liquid inks used for flexographic and gravure printing. These latter inks often comprise 80% solvent and 20% pigment/binder mixture, so that four lbs. of solvent are vented to the atmosphere for each lb. of ink printed. Also, the solvent recovery units often allow some solvent to escape to the atmosphere. Thus, even the relatively small proportion of solvent that escapes is substantial in view of the considerable quantities of printing inks used.
A large volume of plastic film and foil is printed for packaging applications such as bread wrap. The inks used comprise polyamide, nitrocellulose, and other cellulosic polymer vehicles formulated with pigments (20%) and dissolved in organic solvents (80%). Generally, the plastic films are treated with corona discharge, exposed to solvents or flames, or etched with acids to form surface functional groups that enhance the adhesion of the inks. When printed, these inks give adequate, but not outstanding adhesion to the plastic film and foil, even at high humidity.
At about the turn of this decade, the Environmental Protection Agency asked printers and converters who use flexographic and gravure printing to substitute water-based inks for the solvent-based inks presently used. The current water-based inks work reasonably well for printing on absorbent substrates such as paper, paperboard, boxboard, and cardboard, and are widely used for these purposes; however, these inks do not adhere well to the smooth, nonporous, nonpolar plastic film and foil substrates; typically, the adhesion of the ink is poor, especially after exposure to high humidity. Thus, there were strong efforts to develop water-based inks that adhere well to these substrates.
Water-based inks for flexographic and gravure printing have been used for over thirty years but all contain from about 10% to 20% of a water-miscible solvent. In pigmented printing ink compositions, the pigments are ground in a pigment grinding vehicle; the pigment dispersion thus formed is diluted ("let down") in another vehicle. The pigment grinding vehicles are usually water-soluble polymers; the letdown vehicles are usually water-reducible or water-dispersible polymers prepared by emulsion polymerization using high concentrations of carboxyl-containing monomers, followed by neutralization. The latex is required to give an ink film with good film properties, particularly toughness. The latexes comprise high-molecular-weight polymers that give good film properties; the solution polymers are limited to low-molecular-weight polymers because of the viscosity requirements. Again, as described above, these inks performed reasonably well on absorbent substrates but not so on smooth, nonporous, non-polar plastic film and foil substrates.
Prior art illustrative of the foregoing ink and coating compositions include the following: U.S. Pat. No.3,048,530 describes a polyvinyl acetate latex paint, U.V. light is used during polymerization to accelerate the formation of the latex product. Photopolymerizable printing ink compositions are described in U.S. Pat. Nos.3,801,329; 4,003,868; 4,014,771; 4,035,320; 4,056,453; and 4,271,258 and U.S. Statutory Invention Registration H304 (Published Jul. 7, 1987). None of the compositions shown in these patents is aqueous-based. While they are volatile solvent-free, except for Publication H304, none teaches the composition as being useful in flexographic or gravure printing. Significantly, the H304 disclosure teaches the great desirability of the use of a primer coating of polyvinylidene chloride or a solvent-reduced resin solution (e.g. polyester, vinyl, acrylic ester, or cellulose acetate butyrate). Water-based, radiation curable compositions are described in U.S. Pat. Nos. 4,339,566 (textile printing); 4,360,541 (self-pigmented, white, opaque protective coatings) and 5,045,435 (for dip coating and screen printing). None of these water-based compositions is taught as having utility in gravure or flexographic printing.
U.S. Pat. No. 5,028,262 discloses ink compositions which are aqueous dispersions of a water-dispersible polymer, a disperse dye and a dihydroxy benzophenone, the latter functioning to stabilize the dye with respect to precipitation.
U.S. Pat. No. 4,101,493 discloses compositions useful "for paints, adhesives, ink, fiber treating agents, paper treating agents, leather treating agents and the like" comprising "an emulsion of a cross-linking type" containing a mixture of an oligoester-(meth)acrylate with at least 2(meth) acryloyl groups per molecule"with an oil-in-water polymer emulsion adapted to cross-link during final curing. Production of films with thicknesses ranging from 30 to 1,000 microns are disclosed, with no disclosure of any possible use in a flexographic or gravure printing process requiring extremely thin films as low as 2 microns thick, or use of a thickener to provide a viscosity of 10 to 50 poises and rheological properties capable of producing such thin films, and concurrently facilitating the coating or printing step on the defined substrates.